The present invention relates to an apparatus for converting the resolution and gray scale of document image data, and particularly to an apparatus which is capable of converting the resolution and gray scale of input image data to be suitable for diverse functions of document image input/output apparatus such as an image scanner, a facsimile, a digital copier, and the like.
Resolution and gray scale are important factors in determining the image quality in document image handling apparatus such as image scanners, facsimiles, digital copiers, etc. In a conventional document image input/output apparatus such as these, the resolution is represented by the unit of dots per inch (dpi) which indicates how many pixels can be displayed per inch, and the gray scale determines the degrees of brightness for each pixel. The gray scale is determined by the number of data bits per pixel. When there is 1 bit per pixel, the gray scale has two tones of black and white; for 4-bit data, there are 16 graduations of brightness; and for 8-bit, 256 gradations. However, for a pseudo half-tone (dithering), the tone undergoes a pseudo-expansion of an area gray scale, even though it is 1 bit per pixel.
Generally, higher resolution and higher gray scale obtain an image data of good quality. To attain either of these, the number of data bits must become larger. Thus, it is increasingly necessary to vary the resolution and/or the gray scale in accordance with the purpose for which the image data is used as well as to vary the characteristics of the apparatus which handles the data.
For example, a document image may be inputted using an image scanner and the input image is outputted by a printer such as a laser beam printer or the like, which printer can represent just two tones (black and white) per pixel. In this case, the gray scale must be 1 bit per pixel, i.e., two tones (0 or 1), while the resolution varies from 180 dpi to 240 dpi, 300 dpi, 400 dpi or more depending on the printer's performance. Therefore, the image scanner may read an image resolution which equals the output resolution of the printer.
On the other hand, when the input image from an image scanner is outputted to a graphic monitor, the resolution of the graphic monitor is 1,000 pixels or thereabouts per line, and the gray scale varies as 2 tones, 16 halftones, or 256 halftones, etc. As a result, in the above-stated case, the image data is received and outputted under conditions where the resolution and gray scale correspond to those of the monitor.
In a document image input device including the image scanner, the limited performances of the conventional related apparatus such as the laser printer and monitor, merely adopt several modes of the resolution and gray scale and accordingly it is comparatively easy to process the interfacing data. However, with the trend towards diverse system functions and related apparatus, as well as various subdivisions of the uses for the image data, it becomes necessary to control the resolution and gray scale by a series of steps, In addition, the resolution and gray scale have been controlled using software, but the amount of document image data is too voluminous, thus requiring too much processing time and the resolution and the gray scale are not converted at the same time, thereby requiring complicated processing steps which degrade the system's efficiency.